Due to their excellent performances in creep resistance, corrosion resistance, heat resistance, etc., various kinds of functional non-conductive brittle materials have been more and more widely applied in different fields and gradually become an important aspect in nowadays high-tech development. However, due to the hard and brittle properties of materials, some factors in connection with the machining apparatus or external environment would cause locally drastically increased material removal amount, which brings largely increased difficulty in machining with high precision, efficiency and reliability, particularly in the fabrication of micro devices that are needed in micro-electromechanical system (MEMS).
To meet the increasing demands for non-conductive micro devices in the current technological development, there is an urgent need for further developing a micro-machining technique that enables high machining precision and efficiency and high surface integrity after machining. When being applied in the process technological field, the conventional machining using mechanical energy is frequently restricted by the tool size and the brittle property of the material being machined, and therefore not easy to meet the required size and precision. On the other hand, there are some other non-conventional machining techniques that utilize heat energy, electric energy and chemical energy to achieve the required precision and machining size that could not be achieved through conventional mechanical machining. These non-conventional machining techniques have already become a main stream for precision machining of non-conductive brittle materials. Some examples of the currently available non-conventional machining techniques that are often used in micro-machining of non-conductive brittle materials comprise ultrasonic machining, laser-beam machining, ion beam machining, etching machining, etc. However, these methods have their inherent problems. For instance, the laser-beam machining is restricted by its high cost and somewhat low machining quality, the ultrasonic machining is restricted by its low machining rate and limited machining shapes, and the chemical etching machining, while having relatively high machining precision, is restricted by its complicated process and can not be used to manufacture high aspect-ratio devices.
Compared to the above-mentioned non-conventional machining techniques, electrochemical discharge machining (ECDM) is one of possible answers that receive wide attention from different industrial fields. Initially, electrochemical machining is widely applied in metal precision machining and forming. However, when proper machining conditions are given for the electrochemical machining to combine with electric discharge, the hybrid machining technique can also be used to machine non-conductive materials effectively. According to the mechanism of ECDM, the workpiece, tool electrode and auxiliary electrode are immersed in an electrolyte solution (typically alkali electrolyte solution) together. The auxiliary electrode has a larger surface than the tool electrode. Both the electrodes are connected with a DC power source. When a negative voltage is applied to the tool electrode and a positive voltage is applied to the auxiliary electrode in the electrolyte. H2 bubbles are generated at the tool electrode and O2 bubbles are generated at the auxiliary electrode. When the applied voltage is higher than a critical voltage, the generation rate of H2 bubbles will be higher than the rate of the bubbles floating to the liquid surface and H2 bubbles are coalesced into a bubble film around tool electrode surface. Due to the bubble film blanketing effect, the current density of the tool electrode exceeds the critical value. Therefore, the discharge happens between the tool electrode and the surrounding electrolyte. As the workpiece is placed near the vicinity of the tool electrode, the high-temperature heat from the discharge sparks melts the material of the workpiece and speeds the chemical etching by the electrolyte to thereby achieve the purpose of removing away some of the material.
Recently, ECDM has been proven to have good potential for use in the non-conductive brittle material micro-machining process. In the implementation of ECDM, it is the machining precision and efficiency thereof that makes ECDM process practical for use. There are many researches on machining characteristics. In an article written by R. Wüthrich et al and published in Journal of Micromechanics and Microengineering, Vol. 15, S268-S275 in 2005, it was mentioned that, among several influential factors in ECDM, comprising the temperature, concentration, and compositions of the electrolyte, the surface area of electrodes being immersed in the electrolyte and the stability of bubble film surrounding the electrode, the stability of bubble film structure is the main and key factor. In the experiments conducted by R. Wüthrich et al, it was found the thickness of the surrounding bubble film forms a restriction to the minimum range of machining. That is, the minimum range can be obtained after the machining is equal to the thickness of the surrounding bubble film. And, R. Wüthrich et al suggested the adding of a surfactant into the electrolyte to reduce the interface energy between the electrodes and the electrolyte, so as to reduce the thickness of the surrounding bubble film and upgrade the machining precision. However, to recycle the waste electrolyte containing the surfactant, complicated procedures are involved. Moreover, in drilling a microhole using ECDM, for example, when a specific working depth has been reached, it becomes more difficult to expel the bubbles from the microhole, and the electrolyte could not be easily supplied to the end surface of the tool electrode. As it is known, the machining speed is mainly affected by the conversion of discharge performance into the circulation of electrolyte in the microhole. In lacking of sufficient electrolyte, electric discharge will not occur at the end surface of the tool electrode, resulting in slowed machining efficiency. R. Wüthrich et al mentioned in their another article published in Journal of Micromechanics and Microengineering, Vol. 16, N28-N31 in 2006 that when an ultrasonic oscillator is mounted to the electrode holder to cooperate with proper tool amplitude and frequency, it would be able to enhance the flowing of the electrolyte and the debris removal effect in the microhole and accordingly, upgrade the machining efficiency. However, it is still a big challenge to modify the electrode holder while maintaining the stability of the spindle mechanism.